The present invention relates generally to methods of installing offshore constructions such as platforms, wherein a number of supporting columns have bases which rest or are fixed on a seabed, and a platform is supported at the tops of the columns above the waters' surface. In particular, the present method relates to the joining of a number of column sections in situ to form finished columns of desired height based at selected positions on the seabed.
Until now, fixed offshore platforms for exploration or production of oil or gas, have been basically of two different kinds, namely, metal jackets and gravity structures.
Metal jackets are set out on the sea bottom and secured by means of piles which are either driven, or drilled and driven down to depths of about 100-150 meters. For waters up to 300 meters deep and platforms emerging about 30 meters above sea level, the total length of the piles may therefore exceed 480 meters each. Anchoring of piles of such length requires very heavy and costly pile-driving hammers.
Some of the limitations in the use of metal or steel jackets are as follows:
The need for careful previous reconnaissance undertakings, so as to obtain the necessary data for the correct design and installation of the platform. An initial geophysical reconnaissance of the site requires coverage of a considerable area because of the uncertainty of the final position of the structure. A careful bathymetry is needed, to determine the gradient and irregularities of the relief (mounds and hollows) of the seabed. Careful use of an echo sounder, side-scan sonar, penetrometer, magnetometer, and the like is necessary. Also, seismic prospection is a must, to verify the continuity of the strata and to detect the possibility of geological accidents.
Such reconnaissance undertakings sometimes take two or more years before the structure can be designed, and a very precise delimitation of the site selected to install the platform must be performed. Due to the difficulties in determining with reasonable accuracy the bearing capacity and pullout force of each pile, overdimensioning of the structure (both in number of piles and their diameter and length) is not uncommon, in order to obtain a satisfactory safety factor. Nevertheless, the effect of lateral forces (i.e., waves and currents in the submerged portions, and winds in the emerged parts), liquefaction of the soil under the driving force and recovery, scour near the piles and a considerable concentration of the loads owing to the short number of the piles, results in fairly significant movements of the structure which create a dangerous condition for linking elements such as conductors, risers, drilling strings and the like.
A coastal yard for fabrication of the jacket, equipped with heavy cranes to handle the large structure and to put it afloat on expensive barges, raises the cost of the finished platform by a significant amount. Moreover, towing the jacket to the final location, launching and positioning the heavy structure onto the selected final site, are very costly and time consuming operations which can only be performed under favorable weather and oceanographic conditions during short periods of the year.
Other limitations on the use of steel jacket platforms arise in that installation loads (during launching and upending the jacket) are higher than the loads prevalent during the entire life of the structure, and yet determine overdimensioning of many important members of the structure thereby increasing unnecessarily the cost of the platform. Also, cathodic protection of all the structure is required in order to prevent corrosion, and, once the jacket is installed, construction of oil or gas pipelines is needed before exploitation of the platform can begin. Further, the distance from coastal storage facilities, or existing pipelines, limits the feasibility of the metal jacket platforms.
Fixed offshore platforms of the gravity structure type are generally built of reinforced or pre-stressed concrete, or with a concrete base carrying steel lattice structures. Such gravity structures are used as production platforms or as tanks for storage at sea, and are built on a deep water coastal site, and then towed to the immersion point. The structures are made stable by gravity on account of the dimensions and weight of the base (generally of concrete), so that no anchoring is required at the bottom.
Prior to installation, reconnaissance of the sea bottom must be carefully carried out. Bathymetry must yield a highly precise description of the topography of the bottom including flatness and horizontality; seismic prospecting requires rigorous interpretation to determine geological anomalies which may exist near the surface of the seabed; and geotechnical reconnaissance including coring and in situ measurements such as penetrometry down to depths of 20 to 30 meters, and deep coring to about 100 meters, also are required.
Installation of a gravity structure thus is governed by the topography of the sea bottom and the mechanical characteristics of the surface soil. Placement of the structure raises, among other problems, that of contact stresses between the soil and the foundation slab due to irregularities of the sea bottom. Such contact stresses depend on the topography of the site and the characteristics of the soil. This calls for an accurate knowledge of the topography and formulae for the bearing capacity and elastic deformations of the soil.
In practice, it is not possible to obtain a highly accurate map of the irregularities of the soil surface, so that the structure is invariably designed for some degree of broken surface conditions at the bottom. Accordingly, the foundation slab is always overdimensioned.
Usually, gravity structures are designed to be located only on highly consolidated soils such as sands and clays. It is required that the height of the domes of the seabed be no more than a few tens of centimeters and the slope of the bottom be less than 1%. Too great a soil settlement under the effect of repeated loads, tends to ruin the foundation slab and, therefore, damage the structure and linking elements such as conductors, risers and the like.
When transported from a coastal construction dock to the immersion site, a gravity structure may be subjected to unexpected bad weather or oceanographic conditions so that the structure also must be overdimensioned to prevent any damage along such hazardous towing trip.